1. Field of the Invention
The present invention relates to a slitter apparatus for slitting a sheetlike object in a predetermined width, and a method of producing an electrode with the same.
2. Related Background Art
With recent reduction in size and weight of electronic devices such as various office automation equipment, VTR cameras, and cell phones, there are demands for reduction in size and weight and for enhancement of performance of electrochemical devices such as secondary batteries and electrochemical capacitors used as drive power sources for these electronic devices.
Electrodes of the electrochemical devices are normally produced as follows: an electrode active material is mixed with a binder to prepare a coating solution for formation of electrodes, this coating solution is applied onto one surface or both surfaces of a collector, the coating solution is dried to form an active-material-containing layer on one surface or both surfaces of the collector to obtain a sheetlike electrode, thereafter the sheetlike electrode is rolled, and it is cut in a predetermined size.
The cutting operation in production of electrodes is normally comprised of a slitting step of cutting the sheetlike electrode along the direction of flow of production and in a predetermined width, and a clipping step of cutting the electrode of the predetermined width in a predetermined length.
The cutting of the electrode in the slitting step is usually carried out with a slitter apparatus of a shear method, a gang method, or the like. This slitter apparatus has a rotary shaft provided with a plurality of circular upper blades at predetermined intervals, and a rotary shaft provided with a plurality of circular lower blades at predetermined intervals while the two rotary shafts are arranged in parallel with each other and at such a spacing as to achieve a predetermined engagement depth with contact between the upper blades and the lower blades. The slitter apparatus is arranged to be able to slit the sheetlike object in the predetermined width by shear force with rotation of the two rotary shafts.
Here the shear method is a method using round blades having the included angle of 85-90° and the thickness of less than 1 mm, as the upper blades. On the other hand, the gang method is a method using as the upper blades, round blades having the included angle of 90° and the thickness larger than that of the upper blades in the shear method.
The slitter apparatus is the apparatus that cuts the sheetlike object as an object to be cut, by shear force, but is not one that cuts the sheetlike object by sharp blades. For this reason, when the conventional slitter apparatus with the upper blades of the shear method or the gang method is used to cut the sheetlike electrode, there arises a problem that burrs of the collector are likely to be made at cut surfaces. A conceivable reason for it is that the cutting load from the cutting edge is scattered because of the existence of the active-material-containing layer formed on the collector, so as to result in failing to concentrate the shear force on the collector, this results in stretching the collector of metal, and burrs are produced when the collector is cut in this state.
In the case of lithium ion secondary batteries among the electrochemical devices, for example, rolled copper foil is used as the collector in the anode, and if this anode is produced by cutting the copper foil with the slitter apparatus, burrs will be produced at cut surfaces of the copper foil as described above, or cutting chips (copper powder) of the copper foil will be produced. A large burr projecting above the active-material-containing layer of the anode can break a separator and cause a trouble such as a short circuit between the anode and cathode inside the battery. In addition, the copper powder attached to the anode surface or to the edge part of the anode could also cause a similar trouble, and a cleaning step for removal of the copper powder used to be required after the cutting with the slitter apparatus. This resulted in complicating the electrode production steps and posing the problem of increase of production cost.
In order to remedy this problem, for example, Japanese Patent Application Laid-Open No. 11-144713 describes the cutting apparatus of the shear method with the upper blades having the included angle of 25°-65°. The apparatus is intended for suppressing production of burrs in cutting of the anode with the collector of copper foil through the use of such upper blades.
However, where the above-described cutting apparatus with the upper blades was used in cutting the cathode with the collector of aluminum foil, it was difficult to adequately suppress production of burrs and adhesion of aluminum to the side faces of the upper blades.
In the case of electric double layer capacitors among the electrochemical devices, aluminum foil is often used for the collector in the both anode and cathode, and thus the above problem more prominently affects characteristics of the electric double layer capacitors.
Japanese Patent Application Laid-Open No. 8-45500 describes a method of filling a substrate of a three-dimensional metal porous body with an active material and cutting it in a predetermined size, or cutting the above substrate in the predetermined size and filling it with the active material, and thereafter rolling or scraping off the periphery of the substrate to produce an electrode. This is a method of pushing the burrs produced during the cutting, to the inside, or scraping off the burrs, and making the edges of the electrode thinner, thereby preventing the burrs of the electrode from penetrating the separator and causing an internal short circuit.
In the method of rolling the periphery of the substrate after the cutting, however, a pointed burr of needle shape produced during the cutting is once forced down, but it again stands up thereafter. This burr sometimes caused an internal short circuit of the battery. In the case of scraping off the burrs from the periphery of the substrate after the cutting, a pointed chip, and the active material scraped off together with the substrate sometimes attached to the electrode surface, and caused an internal short circuit.
Furthermore, Japanese Patent Application Laid-Open No. 5-190200 describes a method of coating the peripheral edge of the electrode, together with the cut end faces, with a thermally adhesive resin, in order to remedy the problem of burrs and chips.
However, this method is also the method that covers the cut end faces with the resin after the cutting, but not one that suppresses the production of pointed burrs and chips. Therefore, there were cases where burrs or the like once forced down again stood up thereafter to penetrate the separator, and it was difficult to adequately prevent occurrence of the internal short circuit.